thermal ammeter



July 14, 1953 w. N. GOODWIN, JR 2,645,756

THERMAL AMMETER Filed Feb. 17, 1950 Patented July 14, 1953 rios '2545356' THERMAL AMMETEB William Nelson Goodwin, J.,"Newark, N. J., as-

signor to Weston Electrical Instrument Corporation, Newark, N.J., a-cnrporaton of New Jersey Application February' 17, 135D, serial No. 144,674

VThe present inventionV relates" in general to elecutes after the current is 'applied to reach 9ll% of its linal temperature, the meter is especially suited for the measurement'of current in `power cables, transformers, motors, etc. which require a corresponding long time to heat. Such informa;- tion is necessary because the maximum safe electrical loading of most electrical apparatus is directly related to' its temperature rise above ambient and is limited by it. Y

Thermal meters of the type described include a heat storage unit which comprises a heat receiving element having a temperature responsive, element therein, a heat transfer system surrounding the heat receiving element and a 'heater element such as a coil or the like surrounding" the heat transfer system for supplying thermal energy to the system and which is energized either with the same loadcurrentas carried by the ap-z' 'ing element and heat transfer system are so con-f' structed that even for a constant current the temperature of the temperature responsive element does not reach the temperature of the heater element until after the desired long time delay and, for a fluctuating current, the temperature of the thermally responsive element Within the unit is not a measure of an instantaneous value of the current but is a measure of the square root of the average square of the current over the long time delay period. v

An object of the present invention is to provide an improved construction for the heat storage unit of a thermal meter comprised f a temperature responsive element surrounded by heat storage zones having a relatively high heat capacity alternating with heat insulating zones having a low heat conductivity.

A more specific object is to provide an improved heat storage unit for thermal meters featuring a central core made from material having a'rela--y tively large heat capacity and within Whichaa temperature responsive element is placed, the core being surrounded by a tubular body also ,of large heat capacity and separated therefrom by an air space or other matter having 'a yvery low thermal conductivity. If desired the' outer body ilu l I Y 2 of large heat capacity may in turn be surrounded by still other bodies'fof large heat capacity each separated from the other by an intervening lzone of air or its equivalent in terms of heat transfer characteristic. However the principle is the same inv both cases namely that of arranging a plurality of spaced layers or strata of masses having alarge heat capacity, in the path of heat transfer'between the source of heat and the thermally responsive meter element,` and which are sep-14 arated from one another by a zone of low heat conductivity.

Another object is to provide a thermal meter of the time lag type and which is made self-compensating for changes in ambient temperature through use of a dummy heat storage unit having the same heat transfer characteristic as the one with which the heater element is associated, and' also containing a temperature responsive element, by which the meter indication is a function of the diierence in temperature between the two temperature responsive elements.

KVStillan'other object is to provide an improved thermal meter constituted by a pair of adjacently disposed heat storage units only one of which has a heater unit associated therewith, the other serving as a dummy compensating for changes in ambient temperature, and a thermocouple unit for the temperature responsive element, the ""cold junction of the thermocopule being located in the "dummy heat storage unit and the hot junction in the unit with which the heater unit is associated.

vThe-foregoing as well as other objects and advantages will become more readily apparent from the following detailed description of the preferred structural embodiment of the invention and the accompanying drawings in which: 1 Fig. `1 is a top plan of the complete device;

Fig. 2 is a. View in central longitudinal section of one of the heat storage units taken on line 2-2 of ligf;l

Fig. 3 is a transverse section taken on line 3- 3 of Fig. 2; and Y Figs; 4 and 5 are exploded views in perspective ofthe various components which when assembled make up one of the heat storage units.

' "Referring now to the drawing, the heat storage unit l0 includes a cylindrical core Il vhaving' a relatively large heat capacity. The core may bevmade entirely from a metal or other material having ar high specic 'heat such as one of the 'stainlessfsteels or'as` illustrated in the drawing can be comprised of an inner glass tube 'Z- incased within a cylindrical shell 'i3 of polished jections I'I which round off to a line contact vzI'Ia parallel to the axis of the sub-assembly of-caps.

I6 and core II to provide a heat deterrent 'liner contact. These projections I'I, placed 120 apart;v serve to center the caps I6 withina set of hollow plugs I8 made from the same materialI asucapsm..

The inner core II and outer tube I9, both'hav. ing high heat capacitiesv and being coextensive in length, arethus separated from each other by an annulus 2e of airwhich has a very low heat conductivity. Plugs It are likewise provided with 120o spaced projections 2i.which round offl to a rline contact 2 Ia parallel to the axis of cylinder I9, 4and .which are also apertured at 2lb to re. ceive tie bolts 22 which secure the-plugs I8-'withinthe cylinder I 9. Cap I6 and plug I8at the left endof Vthe cylinder areprovided with axial bores 23 aligned with bore I4 in the glass core I2 to,pa-ss thetwo Wires of the thermocouple I5. Moreover, the inner faces ofthe outer plugs I8 are roundedrat Iato minimizethe area in contact with the outer end face of caps IB and thereby minimize heat transfer between the two. Y

The complete assembly of core II, outer tube I9, caps and vplugs I3 ts snugly within another tube 2liv and is centered therein by the 40 three radially extending projections 2| which also establish another air space between tubes Eiland 24. Upon tube 24 preferably of Lava or other ceramic is wound helically in ribbon formanl electrically conductive coil 25 the ends of which are indicated at 25a. Coil 25 constitutesy the-heater unit and is adapted'for use ondirect current or single phase alternating current systems. Formulti-phase systems the single coil 25 would of course be'replaced by a plurality of coils each carrying a current proportional to its associated phase.

The ends of the ceramic tube 24 are closed by plugs 26 which may beof the same material as plugs It. The inner faces ofplugs 26 yare round-,- ed at 26a to minimize the transfer of heat to plugs it and their outer faces are provided with cylindrical bosses 2th that are adapted to enterv correspondingly sized apertures Z'Ia in end supports 2l. YBlug 26v at the left end of tube 24 also has a central bore 26o to pass the thermocouple wires.

' The other heat' storage unit I0 is identically the same construction and both units, held in spaced relation Aby the supports 2l, vare mounted on a suitable'base 28.

l Heat storage unit Ill contains the hot,junc tion Ilia of. the thermocouple, and storage 'unit' l'contains the cold junction I5b. The. thermocouple leads can be brought to terminals 29 on the base 28 for connection to a microammeter 30. Leads 25h from the ends of heater coil 25 can likewise be brought to another pair of terminals 3|. on base 28 for connection to the source of heater current i.

Heat storage unit I 0' constitutes the dummy and hence its dummy coil 25 if one is used remains unconnected, it being noted that While leads 25h from the dummy coil 25 extend to 5 terminals 3I in the same manner as leads 25h from coil 25 and are the same length to the end thatthe-dummystorage unit will Ibe structurally identical withfthe heated unit, the leads 25o are electrically insulated from the conductive termiyfinals 3l by means of insulator inserts 32. Prac- ,tiallymin most cases however the dummy coil `-VV 25'Vwillfbe unnecessary. Unit I0 is placed on base-*28 above ,or-along side of dummy unit I0 relative to the direction of flow of the natural convectioncoolin'giair currents produced by gravity, so that the dummy unit containing the v ,ffcoludj-rjunctionrof, theV thermocouple will be I6, and which t snugly into the ends of an :"affected by ambient temperature only. outer cylindrical tube I 9 of polished stainlessste'e'l or other material having a large heat capacity. 20

.Du'mmy unit I0 not only provides a cold end for thermocouple I5 but also compensates for errors which wouldotherwise result from changes in. ambient temperature, or space temperature variations in the interior of any case within which the complete device may be housed. This compensation .iseifectivefor both magnitude and time-since the two vheat storage units i and Ill being nstructurally identical are alike in both sensitivity-to.temperature and in response time. Therefore both units` are affected exactly alike andA inthe same timebyambient changes, so thatjthe thermoelectric .forces balance exactly for all causesxcept: of course Yfor the heating of` unitIIl by, the current to be measured.

vOperation Aor" the-device is believed to be generallyapparent from the previous detailed Ydescription. Having oriented the base 28 so that as little as possible Vof the heat produced electrically by coil 25 Ais carried by convection currents overA to the dummy unit H3', indicating instrument can be connected as shown to the thermocouple leads and the current i Whose heat ing effectis desired: to be registered can be connected, to terminals 3 I vvvDue to the special construction of the heat storage unit Ill, a temperature change produced bythe. coil 25`will be delayed in reaching the hot junction I5@ of the tiiermocouple since theheat pathina radially inward direct-ion. from coil 2,5 is composed of a plurality of zones havingv a. relatively ,high heat capacity (these being the ceramic cylinder .24, metallic sleeve I9 and core ll)'.,each separated from Vthe other by a zone of low `heat'conductivity (these being the annular air spaces 28 and 29'). Heat transfer in the axialdirection of eaclrunit through the end members 26, I8 and I6 is retarded by the air spaces between them, the inherent heat deterrent characteristic ofthe material from which these enel members are made and the minimum contact area ofV each with the other. The length of the delay is of course determined by the dimensions ofthe device and the number of alternating heat storage and heat deterrent Zones employed.

As, ,explained inthe introduction, a thermal meterlhaving a long time constant is useful in measuring the effective current vwhich determines the temperature of various types of electrical apparatus.l Y For example, in the transmission of power through cables, the power load is limited by the temperature rise ofthe cable and the cable temperature is afunction'of the product of the square of the current` and the time is flows through the cable. yWhen the current uctuates, as is usually the` case, in accordance with fluctuav l2', ,Il the power requirements, it becomes necessary to integrate the heating eiects of the various current values in order to limit the true overall temperature rise in the cable. If for example cables on the average heat under constant current to about 90% of their final temperature in about thirty minutes, a thermal ammeter of the type which has been described designed for the same time constant can thus be used.

The voltage produced by thermocouple unit l and which is impressed upon the conventional microammeter 30 Will therefore be representative not of any instantaneous value of the current passing through heater coil 25 but rather an integration of the current as regards its heating eiect over the time lag constant of the heat storage unit ID. The thermocouple voltage thus becomes a measure of the cable temperature and the scale of the indicating instrument 30 could be calibrated in temperature if desired.

In conclusion, I desire it to be understood that While the herein described embodiment is preH ferred it is but typical of the many structural variations possible Within the scope of the invention as dened in the appended claims.

I claim:

1. A heat storage apparatus for current meters of the type including an electrical measuring device, said heat storage apparatus comprising: a pair of structurally similar heat storage units each of which includes an inner heat storage cylindrical core member having a polished outer surface, an outer heat storage sleeve member disposed concentrically about said core member, said outer heat storage sleeve member being metallic and having a polished outer surface and end members having radial projections supporting said core and sleeve members in radially spaced relation, said radial projections serving to limit the thermal contact area between said core and sleeve members, said core and sleeve members having a relatively high heat capacity and said end members having a relative W heat conductivity; a thermocouple adapted to be connected to said measuring device, the hot junction of said thermocouple being disposed centrally within the core member of one of said heat storage units and the cold junction disposed centrally Within the core member of the other of said heat storage units; and a heater coil surrounding the outer sleeve member of the heat storage unit containing the hot junction of said thermocouple and adapted to be traversed by the current to be measured.

2. Heat storage apparatus of the type dened in claim 1 and Which further includes a nonheated coil of the same construction as said heater coil surrounding the outer sleeve member of the heat storage unit containing the cold junction of said thermocouple.

3. A heat storage unit for current meters of the type including an electrical measuring device, said heat storage unit comprising an inner core of electrical insulating material having a high heat storage capacity, a temperature responsive measuring element disposed Within said core, a first metallic sleeve having a polished outer surface surrounding said core, a second metallic sleeve having a polished outer surface spaced from and surrounding said first sleeve, a third sleeve of electrical insulating material having a high heat storage capacity spaced from and surrounding said second sleeve, end members of electrical insulating material having a low heat conductivity disposed at the ends of said core and sleeves and closing said sleeves, said endme bers including projections supporting said sleeves in spaced relation and limiting the thermal contact area between said sleeves.

WILLIAM NELSON GOODWIN, JR.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Burgess et al., Measurement of High Temperature, pages 151 and 153, 1912 (Photostat in Div. 60.). 13G-4.77.

Bun of Stds. Bulletin, vol 10, 1914, page 335.

13G-44S. (Photostat in Div. 60.)

Industrial Heating, vol. 12, May 1945, pages 776, 136-4AS and '778.

Clark, Iron and Steel Engr, Feb. 1946, page 

